Page 52 - Moving the Earth_ The Workbook of Excavation
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SURVEYS AND MEASUREMENTS
2.2 THE WORK
FIGURE 2.1 Builders’ or contractors’ level. (Courtesy of David White, LLC.)
Vernier. The pointer may be expanded into a vernier such as shown in Fig. 2.2. This is a device
for reading fractions of a scale, which in the example is calibrated to 30″ (half degree) divisions.
The length of 29 divisions on the circle is divided into 30 divisions on the vernier. Each vernier
1
29
space therefore represents ⁄ 30 of a space on the circle and is ⁄ 30 shorter.
In the illustration, if an angle on the main scale is being read from left to right, the zero, or center
of the vernier, shows a reading slightly higher than 2°30″. Reading the vernier to the right, it will
be found that the tenth division line matches exactly with a line on the circle scale. This indicates
10
1
that the zero mark was ⁄ 30 , or one-third, of the way from 2 ⁄ 2 ° mark to the 3° mark, as the differ-
1
ence is canceled out in the course of subtracting the ⁄ 30 difference 10 times.
One-third of 30″ is 10″. The angle is therefore 2°30″ plus 10″, or 2°40″.
FIGURE 2.2 Vernier.
